1. Field of the Invention
The present invention is directed to a sound signal generating apparatus and method for reducing pop noise in a class-D amplifier.
2. Discussion of the Related Art
Audio signal amplifiers used for amplifying audio signals are generally classified into classes A, AB, B, C, and D depending on their operating state during power amplifying stage.
Among the various classes of audio signal amplifiers, the class-D amplifiers are more popular because their efficiency is higher than that of class AB amplifiers and their linearity is also superior due to low cross-over.
Class-D amplifiers are also referred to as switch-mode amplifiers because they resemble switch-mode voltage regulators. A class-D amplifier uses Pulse Width Modulation (PWM) method on an input analog signal or digital PCM signal. This means an input analog signal is modulated by a higher frequency modulation or carrier signal, usually a saw-tooth triangular wave or an input digital PCM signal is converted to a related PWM signal. Upon pulse width modulation, the analog input signal or digital PCM signal becomes discreet or digital with pulse widths used to represent signal strengths of the input original.
The PWM signal presented to the amplifier is a high frequency digital signal with varying widths. A low band pass filter is used to filter the high frequency component to extract the input signal and reduce switching noise.
FIG. 1 shows a waveform of input signals PWMA and PWMB being applied to a conventional class-D amplifier, wherein pulse width modulating signals PWMA, PWMB have substantially the same pulse width but are opposite to each other in phase. FIG. 2 shows a waveform of a speaker voltage VC1 when PWMA, PWMB and power DET1 are applied (Here, DET1 is transited from “Low” to “High” when voltage is turned on). It can be seen from FIG. 2 that a severe overshoot occurs at or near the time when power is applied. The overshoot is caused by the same pulse width of the pulse width modulating signals PWMA, PWMB. This overshoot voltage VC1 at the speaker results in ‘click or pop’ noise.
Therefore, a need exists for an apparatus and a method for generating PWM signals which operate a class-D amplifier but without the pop noise upon application of power or when power is removed from the amplifier.